Estimating biological nitrogen fixation in field-grown soybeans: impact of B value

Abstract

Aim: To evaluate the impact of using different B values on the overall estimation of the proportion of nitrogen (N) derived from the atmosphere (%Ndfa) in soybean (Glycine max L.) field trials under two contrasting management systems via the 15N natural abundance (NA) technique. Methods: First, a literature review of the soybean B value was conducted. Secondly, a greenhouse study was performed to determine the shoot B value by plant development stage for four soybean varieties. Lastly, in two field trials, soybean was grown under two contrasting management systems (common practices, low inputs; and intensified practices, high inputs) and the %Ndfa by development stage was estimated by the NA technique using the three sources of B values (literature review, greenhouse: plant development stage and peak biomass). Results: The average B value reported in the literature for soybean was − 1.80‰, differing on the trial setting (greenhouse − 1.37‰ and field − 2.62‰). The average shoot B value in the greenhouse was − 1.97‰ (R7). The variation between shoot B values measured in this study was small and quantitatively not significant across plant development stages and plant organs. Field trials showed that intensification improved yields and increased maximum N2 fixation rate from 4.9 to 6.2 kg N ha−1 day−1 under irrigated conditions relative to the common practices. Conclusions: The %Ndfa relative to total N demand ranged from 45 to 56%; at the fixation levels measured in this study, the source of B value had relatively low impact on the estimation of %Ndfa.